Melt-spinning (or spunbond processing) is the process of forming fibers by extruding molten polymer through small orifices in a die, collecting the spun filaments on a belt in a uniform random fashion, and bonding the fibers to form a cohesive web. Melt-blowing (or MB) is the process of forming fibers by extruding molten polymer through small orifices surrounded by high speed heated gas jets, and collecting the blown filaments as a cohesive web. This process is also referred to as a blown micro fiber (or BMF) process.
Polyesters such as poly(ethylene) terephthalate (PET) and polyolefins such as poly(propylene) (PP) are two commonly used classes of petroleum based polymers in the commercial production of textile fibers, packaging films, beverage bottles, and injection molded goods by processes such as BMF and spunbond. There is a desire in the market to replace these petroleum-based products with products based on renewable resources. Aliphatic polyesters such as polylactic acid and polyhydroxybutyrate are derived from renewable (plant or microbially based) raw materials but these polymers are typically unsuitable for use in making nonwovens. Despite being commercially available for many years, commercially available spunbond or melt blown products based completely on aliphatic polyesters (e.g. polylactic acid, PLA) are not generally know to exist in the art. Aliphatic polyesters such as poly(lactic acid) (PLA), and webs including such fibers, may shrink up to 40% of the original length when subjected to elevated temperatures due to the relaxation of the oriented amorphous segments of the molecules to relax upon exposure to heat (See Narayanan, V.; Bhat, G. S, and L. C. Wadsworth. TAPPI Proceedings: Nonwovens Conference & Trade Fair. (1998) 29-36).
As mentioned, there is a growing interest in replacing petroleum based polymers such as PET and PP with resource renewable polymers, i.e. polymers derived from plant based materials. Ideal resource renewable polymers are “carbon dioxide neutral” meaning that as much carbon dioxide is consumed in growing the plants base material as is given off when the product is made and disposed of. Biodegradable materials have adequate properties to permit them to break down when exposed to conditions which lead to composting. Examples of materials thought to be biodegradable include aliphatic polyesters such as PLA, poly(glycolic acid), poly(caprolactone), copolymers of lactide and glycolide, poly(ethylene succinate), polyhydroxybutyrate, and combinations thereof.
However, difficulty is often encountered in the use of aliphatic polyesters such as poly(lactic acid) for BMF due to aliphatic polyester thermoplastics having relatively high melt viscosities which yields nonwoven webs that generally cannot be made at the same fiber diameters that polypropylene can. The coarser fiber diameters of polyester webs can limit their application as many final product properties are controlled by fiber diameter. For example, course fibers lead to a noticeably stiffer and less appealing feel for skin contact applications. Furthermore, course fibers produce webs with larger porosity that can lead to webs that have less of a barrier property, e.g. less repellency to aqueous fluids.
The processing of aliphatic polyesters as microfibers has been described in U.S. Pat. No. 6,645,618 (Hobbs et al.). U.S. Pat. No. 6,111,160 (Gruber et al.) discloses the use of melt stable polylactides to form nonwoven articles via melt blown and spunbound processes. JP6466943A (Shigemitsu et al.) describes a low shrinkage-characteristic polyester system and its manufacture approach. U.S. Patent Application Publication No. 2008/0160861 (Berrigan et al.) describes a method for making a bonded nonwoven fibrous web comprising extruding melt blown fibers of a polyethylene terephthalate and polylactic acid, collecting the melt blown fibers as an initial nonwoven fibrous web, and annealing the initial nonwoven fibrous web with a controlled heating and cooling operation. U.S. Pat. No. 5,364,694 (Okada et al.) describes a polyethylene terephthalate (PET) based meltblown nonwoven fabric and its manufacture. U.S. Pat. No. 5,753,736 (Bhat et al.) describes the manufacture of polyethylene terephthalate fiber with reduced shrinkage through the use of nucleation agent, reinforcer and a combination of both. U.S. Pat. Nos. 5,585,056 and 6,005,019 describe a surgical article comprising absorbable polymer fibers and a plasticizer containing stearic acid and its salts. U.S. Pat. No. 6,515,054 describes a biodegradable resin composition comprising a biodegradable resin, a filler, and an anionic surfactant.